In a wind turbine it is essential to control the orientation of the wind turbine rotor in relation to the wind—the so called “yaw”. A conventional wind turbine has a nacelle, mounted on a tower, where the nacelle can be appropriately rotated towards the wind via a yaw system. The yaw is also known as the azimuth. The nacelle has a generator connected via a shaft to the wind turbine rotor, enabling the yaw drive to appropriately control the orientation of the rotor. The optimal performance of the wind turbine is obtained when the rotor shaft is parallel with the wind direction, which maximises the power output and/or minimises the loads. If this optimal position is not obtained the wind turbine is said to have a “yaw error”. The yaw error is expressed as the size of the angle it deviates from the optimal position.
Conventionally, one or more wind vanes and/or sonic wind sensors are placed on the wind turbine nacelle behind the rotor, where their wind direction measurements as electronic signals are communicated to the wind turbine control system. Based on these signals, the control system activates the yaw drive motor and adjusts the turbine yaw accordingly. Mechanical wind vanes and sonic wind sensors can be used and are known in the art.
The position of the wind vanes and/or sonic wind sensors on the nacelle behind the wind turbine rotor is far from ideal, as they will measure the wind after it has passed the rotor, when the wind turbine is in operation. The measurements are therefore heavily influenced by the turbulence generated by the rotor as well as by other aerodynamic effects caused by the nacelle. In addition, buildings, trees, and neighbouring wind turbines can significantly influence the wind direction readings depending on the wind direction. This means that the wind vane and/or sonic wind sensors will report incorrect information to the wind turbine control system and hence produce a yaw error. This yaw error depends on a plurality of factors, among them; the wind speed, the wind direction, the landscape, the design of the wind turbine, the design of the wind vane and/or sonic wind sensors, as well as the wind turbine control system yaw algorithm.
Accordingly, even a correct calibration of the wind vane and/or sonic wind sensor will, at least under certain wind conditions, impose a yaw error on the wind turbine and therefore not make the wind turbine perform optimally.